A rigid, portable shelter designed for the protection of personnel/equipment from the environment, with sliding, folding and/or accordian fold walls, floor(s) and ceiling(s) which permits the expansion of the structure beyond that required for transporting. It is equipped with the capability for transporting by air, land, or sea. It may include leveling devices, lighting, electrical systems, and environmental control connections. The shelter requires minimum site development and no specialized set-up equipment. Excludes BUILDING, (as modified).